Many measurements of interest are generated by applying a repetitive signal to a device under test (DUT) and measuring the frequency response of the output of the DUT. In one common configuration, the output of the DUT is down converted in a mixer to an IF signal that is digitized by an analog-to-digital converter (ADC). The ADC output is then transformed using a Discreet Fourier transform such as a Fast Fourier transform (FFT) to provide a measurement of the amplitudes and phases of the tones in the output signal. The tones will be separated by a frequency that is determined by the repetition rate of the input signal. If the measurement is repeated, the amplitude of the tones and their frequencies will remain the same to within the experimental errors. Hence, spectra that depend only on the amplitude of the tones can be compared from time to time.
The phases of the tones as a function of frequency depend on the starting time of the sample sequence digitized by the ADC relative to some fixed starting point of the repetitive sequence. If this time changes, the phases as a function of frequency also change. Hence, comparing two phase measurements taken at different times presents significant challenges.